Illite was a term proposed in 1937 by Grim et al. as a “group” name for the mica-like, clay-size mineral constituents in argillaceous sediments. Bailey et al. (1984) indicated that illite, as a species, must meet the following characteristics: (1) the structure is not expansible, (2) the 2:1 layer is dioctahedral, and (3) there are compositional criteria. Rieder et al. (1998) gave a representative formula and typical ranges as: K_0.65Al_2.0v(Al_0.65Si_3.35)O₁₀(OH)₂ where ^viR²⁺/(^viR²⁺ + ^viR³⁺) < 0.25, v = vacncy, and ^viAl/(^viAl + ^viFe³⁺) > 0.6 and this differs from muscovite, a closely related mica with an end-member formula and ranges of KAl₂(AlSi₃)O₁₀(OH)₂ where ^ivSi: 3.0 – 3.1, ^viAl: 1.9 – 2.0, K: 0.7 – 1.0 (interlayer cations > 0.85), ^viR²⁺/(^viR²⁺ + ^viR³⁺) < 0.25, and ^viAl/(^viAl + ^viFe³⁺): 0.5 – 1.0. The interlayer deficiency observed in illite is considered an important part of the definition. Rieder et al. defined illite as a “series” name, and series names designate that additional research may be warranted. The clay-size aspects and an occurrence in argillaceous sediments are not considered acceptable criteria to define a mineral. When illite is being referred to as a mineral species, it is advisable to make a clear statement to that effect. Non-pure illite, i.e., material containing an expansible component, is referred to as “illitic”.